1. Field of the Invention
The present invention relates to a driving circuit of a laser diode and a driving method of a laser diode, and particularly to a driving circuit of a laser diode and a driving method of a laser diode that have dual feedback loop for adjusting current to make the laser diode maintain a fixed extinction ratio under different operation temperatures.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating relationships between output powers, input currents, and operation temperatures of a laser diode. As shown in FIG. 1, if the operation temperature of the laser diode is 25° C., when the input current is a bias current IBIAS1, the bias current IBIAS1 can make the laser diode output an output power P0, and when the input current is a sum of the bias current IBIAS1 and a modulation current IMOD1, the sum of the bias current IBIAS1 and the modulation current IMOD1 can make the laser diode output an output power P1, wherein the output power P1 corresponds to a logic value ┌1┘ of a light signal and the output power P0 corresponds to a logic value ┌0┘ of the light signal, and an average value of the output power P0 and the output power P1 is an average power PAVE. If the operation temperature of the laser diode is 85° C., when the input current is a bias current IBIAS2, the bias current IBIAS2 can make the laser diode output the output power P0, and when the input current is a sum of the bias current IBIAS2 and a modulation current IMOD2, the sum of the bias current IBIAS2 and the modulation current IMOD2 can make the laser diode output the output power P1.
As shown in FIG. 1, because a slope of a characteristic curve of the laser diode under the operation temperature (25° C.) is greater than the slope of the characteristic curve of the laser diode under the operation temperature (85° C.), the bias current IBIAS2 needs to be greater than the bias current IBIAS1 and the modulation current IMOD2 also needs to be greater than the modulation current IMOD1 to maintain an extinction ratio (P1/P0) of the laser diode. For solving the above mentioned problem, the prior art adjusts the bias currents and the modulation currents of the laser diode under different operation temperatures according to a lookup table, wherein the lookup table records relationships between the operation temperatures, the bias currents and the modulation currents. Thus, the prior art may need a large number of memories to store the lookup table, resulting in cost being increased. In addition, another prior art provides a single loop automatic power control to fix an average output power of the laser diode. Although the single loop automatic power control can fix the average output power of the laser diode, the single loop automatic power control cannot make the extinction ratio of the laser diode unchangeable. Therefore, the above mentioned prior arts are not good choices for the laser diode when the laser diode operates under different temperature.